gold (Au)

Gold has always been a symbol of material wealth and, because of its unique qualities, has been used to create art treasures of dazzling beauty, such this Graeco-Scythan vase.

Gold (Au) is a soft, bright yellow, corrosion-resistant element, one of the family of noble
metals, and the most malleable and ductile metal. First found by chance, it was being mined by 3000 BC and was being chemically extracted from crushed quartz (by amalgamation with mercury) by 1000 BC. Gold is found in group IB of the periodic
table and is a transition element.
Gold occurs native, typically as small particles in quartz and pyrite but also as deposits after erosion
of the quartz and sometimes as nuggets. It also occurs in tellurides in
the same minerals. It is recovered by mining, panning, or sluicing, and
is extracted with potassium cyanide in the presence of peroxide or air to
give the [Au(CN)2]- ion or by forming an amalgam,
and is refined with electrolysis.

Gold is not attacked by oxygen or by single
alkalis or acids (except H2SeO4). It dissolves in aqua regia and in cyanide and halogen
solutions and is attacked by fluorine.
It forms mainly monovalent – gold (I), or aurous – and trivalent
– gold (III), or auric – salts. Gold (III) chloride is used
as a toner in photography. For
more, see "Chemistry of gold" below.

Gold is a good thermal and electrical conductor, is generally alloyed to
increase its strength, and is used as an international monetary standard,
in jewelry, for decoration, in dentistry, and as a plated coating on a wide
variety of electrical and mechanical components. It is also turned into
thin foil, only 0.1μm thick, called gold leaf, by beating it between
vellum and animal skins. Gold leaf is used for decorative gilding, lettering
on leather-bound books, and for coating artificial satellites, etc., to
reflect infrared radiation. Gold can also be welded in a thin layer to another
metal (rolled gold).

For most uses gold is too soft, and is combined into alloys with other noble metals, the proportion
of gold being measured in carats. White gold is an alloy of gold
with nickel and sometimes other noble
metals, used in dentistry. The number of 'carats' indicates the number of parts of pure gold in 24 parts of the alloy. for example, 9 carat gold is 9/24 pure gold 15/24 other metal.

Gold in the form of a colloid is sometimes used in coloring glass. The isotope198Au (half-life 2.7 days) is used in radiotherapy; the most
common isotope is 197Au.

atomic number

79

relative atomic mass

196.967

electron configuration

1s22s22p63s23p64s23d104p65s2
4d105p66s24f145d9

metallic radius

144 pm

oxidation states

+5, +3, +2, +1

relative density

19.32

melting point

1,063°C (1,945°F)

boiling point

2,966°C (5,371°F)

Chemistry of gold

Gold is an element of the copper group. Of
its oxidation states, the +5 state is found only in AuF5, and
hexafluoroaurates(V). The common states are +3 (generally square planar)
and +1 (generally linear). Both +3 and +1 states form extensive ranges of
complexes particularly with soft bases. Organometallic compounds are readily
formed. Gold(II) complexes are formed with S-ligands but many compounds
which are apparently Au(II) derivatives contain Au(I) and AU(II). Gold forms
stable metal-metal bonds with other metals and cluster compounds, e.g.,
Au11I3L7 or 8 (L = phosphines) are formed
on reduction.

Gold in dentistry and pharmacology

In dentistry, pure gold is now very rarely used as a filling. Gold alloys
are used for crowns, inlays, and bridges, either alone or veneered with
a tooth-colored material, but increasingly non-precious alloys are being
used. Gold alloys are now only rarely used in the metal framework for partial
dentures, cobalt-chromium alloys being used instead.

Several compounds of gold are used in the treatment of rheumatoid
arthritis. Sodium aurothiomale (Myocrisin) is administered by intramuscular
injection. Common side-effects include mouth ulcers, itching, blood disorders,
skin reactions, and inflammation of the colon and kidneys. Auranofin is
a gold compound administered orally.